feat: pick username

* put new registered social login user to session data
* ask to choose username
* simplify checkUsername functions

server/src/utils/auth.js

@@ -56,7 +56,7 @@ export function isValidUsername(username) {
   if (!usernamePattern.test(username)) return false
 
   // check if the username is between 1 and 32 characters
-  if (username.length < 1 || username.length > 32) {
+  if (username.length < 4 || username.length > 32) {
     return false
   }